1. Field of the Invention
The invention relates to a process for the recovery of metals from used nickel/metal hydride storage batteries, in which, after separating the rare earth metals as double salts from an acid digestion solution of the storage battery scrap and subsequent removal of the iron by raising the pH, the filtrate from the iron precipitation is subjected to a liquid/liquid extraction with an organic extractant for the purpose of recovering further metals, such as zinc, cadmium, manganese, aluminium and residues of iron and rare earths.
2. Description of the Related Art
Nickel/metal hydride storage batteries are electrochemical energy stores which, compared with other storage batteries such as, for example, the lead storage battery or the nickel/cadmium storage battery, have a higher storage capacity. These storage systems are therefore increasingly widespread not only for the mains-independent supply of energy to appliances in entertainment electronics, toys and the like, for example in the form of button cells, but also in vehicle drives in large-size battery casings.
The gastight casing, formed from sheet steel, of nickel/metal hydride storage batteries accommodates one or more positive electrodes, one or more negative electrodes and separators, seals and electrolyte.
A positive electrode is conventionally composed of a support and the active mass. Depending on electrode type, gauzes, expanded metal, perforated metal sheet, nonwoven fabric or highly porous metal foams are common as supports. The support material is nickel or nickel-plated iron. The active mass is a mixture of nickel hydroxide and cobalt oxide and, possibly, cobalt. In addition to binders and conducting agents, the mixture may contain further metals, including zinc, as hydroxides in lesser amounts.
A negative electrode is likewise composed of support and active mass. In this case, the latter is a hydrogen-storage alloy. Hydrogen-storage alloys are intermetallic compounds which can be assigned to a AB.sub.5 type on the basis of the simple starting alloy LaNi.sub.5 or to an AB.sub.2 type in accordance with the simple alloy TiNi.sub.2.
Hydrogen-storage alloys of the AB.sub.5 type for nickel/metal hydride storage batteries are conventionally composed of 45-55% Ni, 30-35% rare earths (REs), 5-15% Co, 2-5% Mn and 1-4% Al.
The valuable content substances of said storage batteries make their recovery from waste batteries a particularly urgent requirement.
Many processes are already known for processing used storage batteries, directed in particular at the acidic lead storage batteries. Of these, the processing methods for lead/acid storage batteries are of little interest here because they are remote from the subject of the invention.
Common to most of the other recycling processes, however, are the preparatory steps which comprise pre-sorting the battery scrap supplied in accordance with certain criteria such as cell size, cell shape (round cells, prism-shaped cells, button cells), that it is comminuted mechanically by shredding, that the iron (and the nickel) originating from the casings is separated from the comminuted and optionally washed scrap material, and that the shredder material is finally dissolved in acid for the purpose of wet-chemical further processing. The metals present are then separated in a sequence of fractionated precipitations and extraction processes from this digestion solution or solution of valuable substances and, finally, recovered by electrolytic deposition.
A very complicated separation process which, because it proceeds from unsorted battery scrap and consequently also has to take substances such as copper and mercury, which are absent in alkaline storage batteries, into consideration is described in German Docket Specification 42 24 884. In this process use is made of a liquid/liquid extraction for the selective separation of Zn ions from a fraction of the digestion solution, an organic extractant being fed into a mixer/settler unit as selection phase. Ni and Cd remain behind in the Zn-free solution.
The liquid/liquid extraction also plays an important part, however, in other known processes which were specifically developed for the waste disposal of nickel/cadmium batteries.
According to PCT Publication No. WO 92 03853, the filtered digestion solution, acidified with HCl, of Ni/Cd battery scrap is freed from Cd by counter-flow solvent extraction with 75% by volume of tributyl phosphate (TBP) and 25% by volume of aliphatic hydrocarbon and the Cd is made accessible to electrolytic deposition by so-called stripping (transfer to an aqueous acidic phase).
European Patent EP-A-585 701 provides a separate extraction of Ni and Cd from a fine fraction dissolved in HCl, which fine fraction is obtained, in addition to a magnetic fraction (Fe and Ni) and a light fragment formed from polymers (from casings, separators, bindings) by air separation.
A recycling process which relates specifically to the recovery of rare earths and transition metals from used alloys is the subject of U.S. Pat. No. 5,129,945. In the latter, a solvent extraction is also carried out for the purpose of separating Fe residues from the digestion solution, which Fe residues have entered the filtrate from the preceding hydroxide precipitation. In this case, Co is left behind in the aqueous phase.
In all these known processes, measures are taken which are intended to isolate the metallic constituents, as far as possible individually, in a systematic separation process and to recover the pure metal from the metal compound separated as salt or hydroxide using a reducing agent. Suitable for this latter step are metallothermic or electrolytic processes.
In this way, the iron is indeed returned to the steel industry again; nickel, cobalt and cadmium each return to the production of the battery manufacturer, but each metal is separated separately so that, to produce fresh electrode material, suitable mixtures composed of the refined materials have to be preprocessed again.